[SI-LIST] Re: 答复: [SI-LIST] Re: USB device cable shield connection
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Xu Shuai <xushuai@xxxxxxxxxxxxxxxxxx>
- Date: Mon, 29 Aug 2011 19:12:03 -0700
A ferrite bead between the signal GND and the enclosure would be quite
dubious as it has the potential to set up a resonant network.
Steve.
On 8/29/2011 6:55 PM, Xu Shuai wrote:
> I have not go through the references, but I guess the recommended series
> ferrite bead is not between the shielding of the cable and the enclosure. It
> should be put between the shielding of the cable and signal GND. The reason
> is that if the connection between the signal GND and the enclosure is not so
> good then maybe there is noise between them. If the cable is connected
> directly to the signal GND the noise will be radiated and if there is one
> ferrite bead between them the radiation will be reduced.
>
> If there are enough grounding points between the signal GND and the enclosure
> close to the interface the noise between the signal GND and enclosure will be
> reduced and it is not necessary to put anything between them.
>
> -----邮件原件-----
> 发件人: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] 代表
> steve weir
> 发送时间: 2011年8月30日 9:26
> 收件人: si-list@xxxxxxxxxxxxx
> 主题: [SI-LIST] Re: USB device cable shield connection
>
> The idea that the cable shield is an antenna for noise if it is
> connected directly to a metallic enclosure, but is not if coupled
> through a capacitor does not defies all reason. Faraday cages still
> work. The absolute lowest EMI results from a solid 360 degree bond
> between the shield and the metallic enclosure.
>
> The series ferrite bead recommendation between case and shield is
> perhaps the most absurd of all the recommendations. A ferrite bead
> increases the impedance between the shield and case, increasing
> radiation efficiency at frequencies where the ferrite bead impedance is
> significant.
>
> The only justification for a capacitor is to block low frequency and DC
> loops. For the capacitor to be effective, its impedance must be low at
> signal frequencies. 4.7nF is about 30 Ohms at 1MHz. The biggest problems
> are going to be with even mode currents on the signal transitions at
> much higher frequencies, so 4.7nF s/b more than enough for USB
> applications. The capacitor adds series inductance that limits the
> shield's effectiveness at high frequency. Low inductance components and
> practices pay-off, even if you have to use more than one capacitor.
>
> Steve.
> On 8/29/2011 5:56 PM, earl albin wrote:
>> Colin:
>> I read the TI, Intel, and Cypress app notes. The Cypress app note is
>> certainly insufficient as a guide. For one, the 4700pF cap as shown and as
>> intended, RF mitigation, is incorrect in that it's really an LRC. Proposed
>> network couples DC (1-MOhm) and depending on the cap, likely up to about
>> 10-20 MHz AC between the shield and signal grounds. The 2-meter cable is not
>> an efficient radiator at those frequencies. Again though this all depends on
>> the cap and the layout. The ferrite bead accomplishes this effect sort of in
>> the opposite direction
>>
>> I thought both TI and Intel were generally in agreement, perhaps not exactly
>> on the values but on the approach. Intel did mention that this was what they
>> found to work. Likely the first attempt didn't then they made
>> adjustments/changes till they got it to pass.
>>
>> So at least you could start with either Intell or TI, better than guessing.
>>
>> On Mon, Aug 29, 2011 at 11:42 AM, Colin D Bennett<colin@xxxxxxxxxxx> wrote:
>>
>>> I have been trying to understand how to ideally handle the cable shield
>>> on a USB device. (Full Speed USB, in this particular instance.)
>>>
>>> As seems to be the case with many signal integrity issues,
>>> contradictory recommendations abound, each with its own unsupported
>>> claims. Even authoritative-sounding sources such as Texas Instruments,
>>> Intel, FTDI, and Cypress Semiconductor seem to disagree on the correct
>>> way to handle the cable shield on USB devices.
>>>
>>> Contrary to my initial supposition, the purpose of the USB cable shield
>>> is not to protect the USB data lines from outside interference, but
>>> rather to prevent the USB device from radiating EMI.
>>>
>>> Here are some of the options that have been recommended.
>>> Note that (2)—series capacitor to pass high frequencies only—seems to
>>> directly contradict (3)—series ferrite bead to block high frequencies
>>> only.
>>>
>>> (1) Connect shield directly to signal ground.
>>>
>>> - “Full speed devices use a shielded cable which requires that the
>>> connector shell be tied to the ground plane.â€
>>> Intel. EMI Design Guidelines for USB Components. Sec 5.4 (p. 9).
>>> <http://www.ti.com/sc/docs/apps/msp/intrface/usb/emitest.pdf>.
>>>
>>> (2) Connect shield to signal ground through a capacitor.
>>> (Possibly with high-value parallel resistor approximately 1 Mohm.)
>>>
>>> - Connect shield to signal ground with 0.01 µF to 0.47 µF capacitor.
>>> FTDI. Debugging FT232BM and FT245BM Designs. Section 3.2 (p. 11).
>>> <http://www.ftdichip.com/Documents/AppNotes/AN232B-06_11.pdf>.
>>>
>>> - Cypress recommends a 1 Mohm resistor in parallel with a 4.7 nF
>>> capacitor.
>>> Steve Kolokowsky& Trevor Davis (Cypress Semiconductor).
>>> Common USB Development Mistakes – You Don’t Have To Make Them All
>>> Yourself! Figure 7 (p. 7).
>>> <http://uk.farnell.com/images/en/ede/pdf/usb_dev_mistake.pdf>.
>>>
>>> - “Tying the shield directly to ground would create a direct path
>>> from the ground plane to the shield, turning the USB cable into
>>> an antenna. To limit the USB cable antenna effect, it is
>>> recommended to connect the shield and ground through an RC
>>> filter. Typically, R = 1MΩ and C = 4.7nF in Figure 3-5.â€
>>> Atmel AVR1017: XMEGA - USB Hardware Design Recommendations.
>>> Section 3.3.3 (p. 8).
>>> <
>>> http://atmel.com/dyn/resources/prod_documents/doc8388.pdf?doc_id448&family_id`7
>>>> .
>>> (3) Connect shield to signal ground through a ferrite bead.
>>>
>>> - “Place a ferrite in series with the cable shield pins near the
>>> USB connector socket to keep EMI from getting onto the cable
>>> shield.â€
>>> Texas Instruments Application Report.
>>> USB 2.0 Board Design and Layout Guidelines. Sec 2.2.4 (p. 3).
>>> SPRAAR7 – December 2007.
>>> <
>>> http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=spraar7&fileType=pdf
>>>> .
>>> Whether or not the device has a metal chassis, and the handling
>>> of chassis ground and signal grounds, (as well as how the USB cable
>>> ground is connected to either one) is certainly important as well, but
>>> this isn't clearly discussed in most of the writings on USB cable shield
>>> grounding.
>>>
>>> The device I'm developing is a bus-powered device which will likely be
>>> in an unshielded plastic enclosure.
>>>
>>> Thanks in advance for any bits of wisdom on this topic full of
>>> contradictory information.
>>>
>>> Regards,
>>> Colin
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